An audio amplifier is used to amplify a weak audio signal to drive an electroacoustic transducer such as a speaker or a headphone. FIG. 1 is a circuit diagram illustrating a configuration of an audio amplifier 100r including a D-class amplifier. The audio amplifier 100r includes a pulse width modulator 110, a first driver 112, a second driver 114, a 1D-class amplifier 116, and a 2D-class amplifier 118. The pulse width modulator 110 pulse-width-modulates or pulse-density-modulates an audio signal S1. Pulse-modulated audio signals (hereafter, referred to as a “pulse signal”) S2p and S2n are input to the first driver 112 and the second driver 114, respectively.
The load, an electroacoustic transducer 2, is bridge-transless (BTL)-connected to the 1D-class amplifier 116 and the 2D-class amplifier 118. The first filter 20 is inserted between a positive electrode terminal (+) of the electroacoustic transducer 2 and an output of the 1D-class amplifier 116, and the second filter 22 is inserted between a negative electrode terminal (−) of the electroacoustic transducer 2 and an output of the 2D-class amplifier 118. Each of the filters 20 (22) is a primary filter having a series inductor L1 (L2) and a shunt capacitor C1 (C2).
The first driver 112 complementarily switches a high side transistor and a low side transistor of the 1D-class amplifier 116 depending on the pulse signal S2p. Similarly, the second driver 114 complementarily switches a high side transistor and a low side transistor of the 2D-class amplifier 118 depending on the pulse signal S2n. 
FIG. 2 is a waveform diagram when an audio output circuit 8r of FIG. 1 performs a differential operation. In the present disclosure, a vertical axis and a horizontal axis of waveform views or time charts are appropriately magnified or reduced to facilitate understanding and also simplified to facilitate understanding of each illustrated waveform.
Here, to facilitate understanding, a case in which the pulse signals S2p and S2n are generated by comparing a triangular wave and the audio signal S1 will be described. In a differential type D-class amplifier, the pulse signals S2p and S2n have an anti-phase. As a result, the output voltages Vo+ and Vo− are differential signals, and an amplitude of the differential signal Vo (=Vo+−Vo−) is two times the source voltage VDD of the 1D-class amplifier 116 and the 2D-class amplifier 118.
In the differential type D-class amplifier, the first filter 20 and the second filter 22 serve as low band pass filters for removing a switching frequency of the differential signal Vo and reproducing the original audio signal S1.
Recently, instead of the differential operation of the D-class amplifier described above with reference to FIG. 2, a filterless operation is employed. FIG. 3 is a waveform diagram when the audio output circuit 8r performs a filterless operation. During the filterless operation, the pulse signal S2p is generated by comparing the audio signal S1 and a triangular wave, and the pulse signal S2n is generated by comparing a reversal signal #S1 of the audio signal S1 and a triangular wave. An amplitude of the differential signal Vo applied to the electroacoustic transducer 2 is ½ of the differential operation. This scheme does not require a low band pass filter for removing a switching frequency, and thus, it is called a filterless operation or a filterless scheme. However, in order to restrain unnecessary electromagnetic interference (EMI), a filter may not be removed, and in the filterless scheme, the first filter 20 and the second filter 22 serve as EMI removing filters.
Regarding the audio output circuit 8r of FIG. 1, the present inventors have reviewed sound quality indices such as (i) total harmonic distortion+noise (THD+N), (ii) inter-channel crosstalk and (iii) noise characteristics etc., and recognized the following technical problems.
Sound quality indices of the filterless type D-class amplifier are significantly affected by a chip layout. This means that the modification or change of a chip layout of an audio amplifier by a semiconductor manufacturer may degrade the sound quality indices. In this case, whenever a chip layout is modified or changed, verification needs to be performed to optimize sound quality indices, which may extend the development period and increase the design costs.